An illustration provided by Nasa shows a cutaway view of Mars’ interior as a meteor strikes one side of the planet’s surface, left, creating seismic waves detected by the space agency’s InSight lander, right. Using data from Nasa’s retired InSight lander, two separate teams of researchers found evidence on Mars of a solid inner core like Earth’s and a mantle full of ancient debris. Photo / Nasa, JPL-Caltech via The New York Times
An illustration provided by Nasa shows a cutaway view of Mars’ interior as a meteor strikes one side of the planet’s surface, left, creating seismic waves detected by the space agency’s InSight lander, right. Using data from Nasa’s retired InSight lander, two separate teams of researchers found evidence on Mars of a solid inner core like Earth’s and a mantle full of ancient debris. Photo / Nasa, JPL-Caltech via The New York Times
Three years ago, Martian dust shrouded the solar panels on Nasa’s InSight lander, preventing further operations and leading to its retirement.
The mission’s data continues to reveal information about the red planet — particularly how it is both like, and unlike, our own.
In a new paper published yesterday in the journal Nature, scientists announced that the innermost part of Mars’ core was not liquid but solid, just like Earth’s.
Another paper, published last week in the journal Science by a different team, described the planet’s mantle as full of ancient debris, with a texture more like rocky road than smooth chocolate ice cream.
Together, the studies offer clues to Mars’ history, including what violent collisions it experienced in the past and how it lost its magnetic field.
The knowledge could help scientists better understand other rocky worlds and identify the conditions necessary for habitability elsewhere in the galaxy.
“There are connections to our own planet, to how planets form in general and to how we understand different planets outside our solar system,” said Ingrid Daubar, a planetary scientist at Brown University and former member of the InSight science team who was not involved in either study.
Earth’s early geological history is difficult to decipher because the crust is ever changing, a feature driven by active tectonic plates that continuously recycle material between the surface and the layer below it.
Mars is different. Its crust is a single, stagnant lid protecting an interior that has been virtually frozen in time since its formation.
A photo provided by Nasa shows the final selfie taken by the InSight lander from Mars’ surface on April 24, 2022, or the 1211th Martian day of the mission. Photo / Nasa, JPL-Caltech via The New York Times
Listening to the tremors of quakes as they travel through Mars is a crucial technique for mapping its insides.
A seismometer placed on the planet by the InSight lander, launched in 2018 by Nasa’s Jet Propulsion Laboratory, recorded more than 1300 quakes over the mission’s lifetime.
Scientists analysed eight marsquakes that generated seismic waves through the planet’s interior at various frequencies. They found that waves with high frequencies arrived at the seismometer delayed, some by dozens of seconds, compared with those with lower frequencies.
Simulations showed that the waves were scattered and slowed by lumps of material that were different from the rest of the mantle, “like sonar pinging through a field of submerged boulders”, said Constantinos Charalambous, a planetary scientist at Imperial College London who led the work.
Waves with higher frequencies have smaller wavelengths, making them more susceptible to deflection off smaller pieces of debris, he explained.
The debris extends as far as 870 miles (1400km) below the Martian surface, with some fragments being over 3.2km wide. The scattering of debris suggests that whatever caused it was exceptionally energetic.
“An impact fits the bill,” said Thomas Pike, an engineer at Imperial College London, adding that a series of violent strikes could also have been responsible.
Those collisions may have injected debris from an asteroid or other projectile into the interior.
That this material survived over billions of years suggests that the mantle is thick and sluggish, preventing the debris from being fully stirred away, Pike said.
An illustration provided by Nasa shows a giant asteroid striking Mars 4.5 billion years ago in an impact that could have caused lumps of debris that now lie scattered throughout the Martian mantle. Using data from Nasa’s retired InSight lander, two separate teams of researchers found evidence on Mars of a solid inner core like Earth’s and a mantle full of ancient debris. Photo / NASA, JPL-Caltech via The New York Times
By contrast, the researchers investigating the nature of Mars’ core studied seismic waves with low frequencies.
With data from more than 20 marsquakes, they discovered that waves travelling through the deepest part of the core had arrived at the seismometer up to 200 seconds earlier than what would have occurred if the centre of the planet had been fully liquid.
The findings suggest that the core has an outer liquid layer surrounding a solid inner layer, said Daoyuan Sun, a seismologist at the University of Science and Technology of China. Quake tremors travelled faster through the solid core, which has a radius of about 375 miles (600km), he said.
Earth’s core is also solid on the inside and liquid on the outside. Its mantle is constantly circulating, pulling heat from the centre of the planet and causing the liquid outer core to churn.
This fuels a magnetic field around Earth that shields us from harmful solar radiation and creates aurorae — colourful displays of dancing lights.
Although its core has a similar structure, Mars lacks a magnetic field.
Douglas Hemingway, a planetary scientist at the University of Texas at Austin who was involved in the study, said that this might partly result from the mantle’s sluggish nature, which makes it inefficient at drawing heat from the core.
So far, Mars is the only planet in the solar system besides Earth that has been studied using seismology.
An upcoming Nasa mission called the Farside Seismic Suite, scheduled to launch in 2027, will aim to measure the interior of the moon using the same technique.
In 2028, the space agency will also launch Dragonfly, a drone equipped with a seismometer, to probe underneath the icy shell of Titan, Saturn’s largest moon.
Knowledge gained from other rocky worlds will ultimately inform what scientists can learn about our own planet.
“You can’t understand Earth without understanding it in the context of how it is similar to, and different from, other bodies,” Hemingway said. “We need to study multiple different kinds.”
